I agree with foxl. You need to go with his theme first. Floortime methods really do a great job of getting the eye contact and establishing the back and forth style of communication even if it is just an exchange of affect and not language.
That's a good suggestion about getting into his theme and then re-directing. I try to answer whatever question ds asked and then say, "You know, I asked you a question too - can you please answer mine?" Then I usually have to repeat the question (since he didn't really hear it) and if he goes back to his topic instead - I point this out to him and tell him that I answered his question and it is respectful to then answer mine. I did this for a long time over the last few years and now usually have much better conversations with him - not always of course!
I do also try to prompt eye contact - but I don't ask him to look me in the eye. I ask him to look at my FACE. I have told him I don't care what part of my face he looks at - it could be my eyebrow, my nose, etc. But, if he is looking at something that he was playing with - we are never going to have a conversation.
Also, there are times it is not worth pursuing in my opinion. If he is particularly stressed, had a rough day, I don't pursue conversations as actively as I might on a better day. I give him some space to de-compress before attempting conversation.
You might try conversations that involve episodic memory (emotional memories with meaning.) Like sitting with your son and looking at a family photo album (or shoebox of pictures.) That kind of thing is conducive to just sharing thoughts together. Or you could look at pictures of a vacation place you've been and you can talk about those pictures. I would really, really try not to ask questions, but instead say stuff like, "Oooo, that's our favorite," or something kind of inviting like that. Just comments that invite (not demand) comments. I, myself, do not see that we get much fruitful conversation out of questions, even with typical kids, most of the time. But that's just me.
When you think about it, a child saying a curt "no" and a child going on and on and on about his own subject are both ways of talking that involve no emotion sharing with the other person. Verbal and nonverbal kids can be conversational - or not conversational at all. Having actual words doesn't have much to do with the desire to converse. If this were me, I would focus on emotion sharing instead of actual conversation, if trying to encourage conversation. If you do get snippets of back and forth conversation, then those are signals that you are on the right track. But I would not target conversation specifically.
IMO, you can't teach conversation. It comes out of both people's ability/interest/willingness to connect with the other person. Conversation is just one way to express emotional connection. In other words, you can't teach emotional connection, but you can do a lot to help a person feel more at ease and familiar with it, and thus, willing to go there. BTW, by "emotional connection" I don't mean slobbery hugs & kisses all the time. Just the feeling of being interested in someone is emotional connection.
Here's a great activity that works on foundational conversation skills. Notice how mom & son negotiate over the lyrics of the song. Mom is leading the activity, but always staying aware of her son. He is regulating his movements to hers. Son is getting a feel for how conversational give & take works:
http://www.youtube.com/watch?v=7-a2qnq-Am8
Floortime.
Or RDI.
ETA: Micki, you should be getting into his theme, (soil) THEN redirecting him to yours ...